usage limit

2weeks after my upgrade i got an sms abt usage limit bt wen i called 808 i was told tht the contract i upgraded on had been open to allow usage limit of R5000 n i have already exhausted R4688 of that money via data in 2weeks bt wat i ddnt understand was hw was it my fault becoz i ddnt open a new contract ,it was an upgrade n worse of all m stuck wit a bill tht i have to pay mean while the mtn agent who assisted me with the upgrade is walking free coz wen i went to head office the lady from complains showed me tht it was the guys responsibility to set the limit coz i dnt get how some one wud set a limit of R5000 mean while thts my monthly salary and now its been 5months and no one is helping me out i dnt knw hw mtn came to the decision that i shud pay ths money mean while it was nt my fault the bill is now R6900 and i dnt think its fair for me to pay for someone elses mistakes meanwhile its affecting me alone m the one stressed every month and im the pne who is forced to pay for their staffs mistakes till today i dnt understand how they came to that decision